The present disclosure relates generally to electron beam sources and, more particularly, to electron beam sources including photocathodes for the generation of electron beams.
High resolution electron beam sources are used in systems such as scanning electron microscopes, defect detection instruments, VLSI testing equipment, and electron beam (e-beam) lithography. In general, e-beam systems include an electron beam source and electron optics. The electrons are accelerated from the source and focused to define an image at a target.
In applications such as electron beam lithography, for example, it is often desirable to modulate an electron beam so as to control a dose of electrons delivered to a pixel on the target. Preferably, the electron beam is modulated to address more than about 100 megapixels per second (corresponding to more than 100 megabits per second in a digital implementation) to thereby rapidly communicate a pattern to the target. Such high pixel throughput typically requires that the electron beam be modulated with a digital signal having (20%-80%) rise and fall times less than about 10 nanoseconds (ns), or an analog signal having transition times between pixel levels less than about 10 ns.
In electron beam sources such as those disclosed, for example, in Aton et al. U.S Pat. No. 5,156,942 and in Baum et al. U.S. Pat. No. 5,684,360, electron beams are generated by light beams incident on a photocathode. These electron beams may be modulated by modulating the light beams. Alternatively, Lee et al. U.S. patent application Ser. No. 09/052,903, filed Mar. 31, 1998, assigned to the assignee of the present application, and incorporated herein by reference in its entirety, and the corresponding Lee et al. International Patent Application Ser. No. PCT/US99/05584, filed Mar. 16, 1999, assigned to the assignee of the present application, published Oct. 7, 1999 as International Publication Number WO 99/50874, and also incorporated herein by reference, disclose photocathodes having gate electrodes. An electron beam supplied by such a gated photocathode is electrically (not optically) modulated by controlling a voltage between the gate electrode and a photoemitter in the photocathode of, e.g., about xc2x15 volts.
Both the gate electrode and the photoemitter in such a gated photocathode are typically operated at voltages of, e.g., tens of kilovolts with respect to ground voltage. In contrast, control systems controlling the gate and photoemitter voltages typically operate at voltages near ground voltage. Consequently, the control systems must be electrically isolated from the high voltages.
High voltage components of electron sources have previously been isolated from low voltage control systems with, e.g., optical isolation systems operating at low signaling speeds. For example, Hartle U.S. Pat. No. 5,808,425 discloses an electron source in which a low bandwidth feedback system employing optical isolation regulates the level of a cathode emission current. Similarly, Fish U.S. Pat. No. 6,072,170 discloses a cathode switch employing optical isolation for an image intensifier tube. This cathode switch is rather slow, having, a turn on time of about 50 nanoseconds (ns) and a turn off time of about 300 ns.
What is needed is an electron source suitable for high speed operation and control, e.g., for lithography.
An electron source is disclosed in which an electron beam is controlled optically by providing a beam of light outside a housing, detecting a portion of the beam of light inside the housing, thereby receiving a signal which is a series of pulses, each pulse having a rise time and a fall time each less than about 10 nanoseconds, and controlling a difference between a voltage applied to a photoemitter inside the housing and a voltage applied to a gate electrode inside the housing in response to the signal. This is in the context of a digital (pulsed) signal. However, it will be apparent that the signal may be an analog signal having an equivalent information carrying capacity, and characterized by the same rise and fall (transition) times.
In a first embodiment, the electron source includes a transmissive substrate, a photoemitter on the substrate, a gate insulator on the photoemitter, a gate electrode on the gate insulator, a housing enclosing the photoemitter and the gate electrode, a light source outside the housing, and a detector in the housing located to receive light from the light source. The detector is electrically coupled to control a voltage applied to one of the gate electrode or the photoemitter.
In one embodiment, the light source includes a laser and an electroabsorption optical modulator located to modulate an output of the laser. Other embodiments include additional light sources, detectors, photoemitters, and gate electrodes. In one embodiment, light output from a first light source is combined by an optical multiplexer with light output from a second light source.
The inventors have recognized that high pixel throughput in, e.g., an electron beam lithography system employing a gated photocathode requires communication of control signals having very short rise and fall times to the photoemitter or gate electrode. The inventors have further recognized that communication of such control signals can be accomplished with optical technology developed for use in telecommunications, such as electroabsorption modulated lasers.
Optical communication of control signals to a photoemitter or gate electrode in an electron source electrically isolates high voltages from ground-referenced low voltage signals controlling, e.g., light sources. Such electrical isolation improves operator safety, and reduces the complexity of designing and fabricating gated photocathode electron sources. In addition, high bandwidth optical communication of control signals reduces the length over which very high frequency and high voltage electrical signals are transmitted to photoemitters or gate electrodes, and thus reduces radio frequency cross-talk between those signals.